Spade's sister, Reta Saffo, told the Kansas City Star newspaper that her younger sibling had suffered from mental-health and substance-abuse problems for years before she and Andy took a break from their relationship.

Mr Spade, her husband of 24 years and longtime business partner, tried to get her to seek help, but "nothing came of it," Ms Saffo said.

Mr Spade has denied this, claiming in an open letter published in The New York Times that Spade had been seeing a doctor for the past five years and was taking anxiety medication but not abusing alcohol or drugs.

He also said they "never even discussed divorce."

However, a source told People magazine that the Spades "both had lawyers and were divorcing. But they hadn't filed yet."

Friends and family said the Spades kept their personal demons in the closet and out of public view.

Her father, Earl "Frank" Bros­nahan, said she seemed fine when they spoke on Monday, the day before her death.

"She was happy and we made plans to meet in California," her dad told The Wall Street Journal.

"Nobody knows what's ­going on with someone. When the doors close, you never know."

Spade's suicide note included a message to her and Andy's 13-year-old daughter, Frances Beatrix Spade, that read: "This has nothing to do with you. Don't feel guilty. Ask your dad," police sources told the New York Post.

Ms Saffo said the pressure of ­celebrity may have gotten to her sister.

"She never expected it - nor was she properly prepared for it," Ms Saffo told CBS. "Unfortunately, untreated, it finally took its toll on her."

In an email to the Star, Ms Saffo said Spade "was always a very excitable little girl and I felt all the stress/pressure of her brand (KS) may have flipped the switch where she eventually became full-on manic depressive."

The stress may have deepened an already growing divide between the couple. Speaking to NPR last year, Spade revealed how her personality differed from her husband's.

"I also am very - a nervous person. I worry a lot. And Andy could not be more different," she said.

"And [I'm] always, you know, the sky is falling. And Andy was like, 'Oh, you know, it's fine.' "

Spade had told People that the secret to keeping their marriage together was to honour their vows.

"We laugh a lot," she told the magazine. "And my daughter has a really funny sense of ­humour."

"But when we need to be serious, we're serious, and she can tell the difference, but I think we've had a lot of fun. And then also, you know, you take the good with the bad. You take those vows seriously. Through good and bad."

Spade's father tried to no avail to convince her to leave the industry and its stresses behind.

"She didn't want to," he told the Journal. "She liked the business."

Ms Saffo blamed Spade's suicide on sycophantic hangers-on, claiming that industry underlings prevented her from seeing how vast her mental-health problems had become.

She was "surrounded by YES people for far too long, therefore she did not receive the proper care for what I believe to be (and tried numerous times to get help for) bipolar disorder," she said in a written statement to CBS.

Designer Kate Spade poses with shoes from her collection. Picture: AP

Other relatives have lashed out at Ms Saffo, claiming the "estranged" sister is just looking for her 15 minutes of fame.

"The family is disgusted and saddened that at this time of great sorrow, Kate's sister, who has been estranged from the entire family for more than 10 years, would choose to surface with unsubstantiated comments," a source close to the family told People about Ms Saffo.

"[Saffo's] statements paint a picture of someone who didn't know her at all," added the source, who described Spade as a "kind, generous, funny, warm and extremely private person."

If you are experiencing mental health issues or suicidal feelings contact Lifeline on 13 11 14 or BeyondBlue 1300 224 636. If it is an emergency please call 000.

This article originally appeared in the New York Post and is republished here with permission